In this paper, the temperature and reaction source gas flow of growing α-Ga2O3 by hydride vapor phase epitaxy (HVPE) were optimized by numerical simulation. Different from the traditional reaction chamber in which Ga source carried by HCl or Cl2, we use the method of external Ga source, which can accurately adjust the component proportion, mole fraction and concentration of GaCl/GaCl3. In addition, the activation energy of the reaction between GaCl3 and O2 was calculated by molecular simulation software Gaussian, and the activation energy of α-Ga2O3 convert to β-Ga2O3 was obtained by fitting the experimental data. On this basis, we researched the growth temperature and the composition ratio of GaCl/GaCl3, and then we concluded the optimal growth conditions of HVPE α-Ga2O3.